Aircraft are known that have undercarriages with wheels that are fitted with brakes having electromechanical actuators. In general, the brakes are powered by power supply units that deliver power to the actuators. The power supply units receive power from at least one electrical power supply bus of the aircraft that serves to convey the power generated by electricity generators that are driven by the jets or by an auxiliary power unit.
Aircraft manufacturers are paying ever increasing attention to the availability of various items of equipment, and in particular in the braking system. Various types of failure concerning the braking system may be taken into consideration:
1) a failure that does not prevent the aircraft being operated and that does not prevent it taking off, and for which a second failure occurring in flight would be without consequence for the safety or the performance of the braking system;
2) a failure that does not prevent operation of the aircraft and does not prevent it taking off, but for which a second failure occurring in flight would lead to degraded performance;
3) a failure preventing operation of the aircraft but that can be repaired during the turnaround time of the aircraft at an airport, enabling the aircraft to depart on time or with an acceptable delay (typically a few minutes);
4) a failure that prevents the aircraft from departing but that can be repaired after a considerable delay, but not requiring the flight to be canceled; and
5) a failure that prevents departure of the aircraft and that would lead to a delay on repair requiring the flight to be canceled.
Only failures of the first type enable the aircraft to be operated without any cost consequence, with it being possible for the faulty equipment to be repaired during programmed maintenance. Failures of the second type are in themselves only potentially limiting, since it is only in the event of a second failure that the performance of the braking system is diminished, thus leading the airline to limit the operating margins of the aircraft (in particular limiting the maximum weight of the aircraft). Failures of the third type have no operational impact but they are expensive, since they require non-programmed maintenance.
Failures of the fourth and fifth types give rise to extra costs associated with indemnifying passengers, and possibly to damaging the image of the airline.
It is therefore desirable to organize the braking system in such a manner that, insofar as possible, it is liable only to failures of the first type. A braking system is thus desired that remains available and operational regardless of the type of failure affecting one or other of the pieces of equipment of the braking system.
In this respect, documents U.S. Pat. No. 6,296,325 and U.S. Pat. No. 6,402,259 disclose braking system architectures that include a certain number of units (EMACS) that power brake actuators fitted to the wheels of main undercarriages, each EMAC controlling a group of brake actuators for each wheel of an undercarriage. In the architectures proposed, the failure of an EMAC is then very troublesome unless it is dealt with immediately. The aircraft can be operated with one failed EMAC. It suffices to call on the actuators that continue to be powered to deliver greater braking force in order to guarantee braking performance. However, if another EMAC that controls the complementary portion of the actuators fitted to the wheels of a given main undercarriage should fail, then the situation would become totally asymmetrical with all of the brake actuators of the wheels carried by that undercarriage then being incapable of exerting any braking action.